of
1807; Norton had applied it to the faint light tracks proceeding from
that of Donati;[1274] Winnecke to the varying deviations of its more
brilliant plumage. Bredikhine defined and ratified the conjecture. He
undertook to determine (provisionally as yet) the several kinds of
matter appropriated severally to the three classes of tails. These he
found to be hydrogen for the first, hydro-carbons for the second, and
iron for the third. The ground of this apportionment is that the atomic
weights of these substances bear to each other the same inverse
proportion as the repulsive forces employed in producing the appendages
they are supposed to form; and Zoellner had pointed out in 1875 that the
"heliofugal" power by which comets' tails are developed would, in fact,
be effective just in that ratio.[1275] Hydrogen, as the lightest known
element--that is, the least under the influence of gravity--was
naturally selected as that which yielded most readily to the
counter-persuasions of electricity. Hydro-carbons had been shown by the
spectroscope to be present in comets, and were fitted by their specific
weight, as compared with that of hydrogen, to form tails of the second
type; while the atoms of iron were just heavy enough to compose those of
the third, and, from the plentifulness of their presence in meteorites,
might be presumed to enter, in no inconsiderable proportion, into the
mass of comets. These three substances, however, were by no means
supposed to be the sole constituents of the appendages in question. On
the contrary, the great breadth of what, for the present, were taken to
be characteristically "iron" tails was attributed to the presence of
many kinds of matter of high and slightly different specific
weights;[1276] while the expanded plume of Donati was shown to be, in
reality, a whole system of tails, made up of many substances, each
spreading into a separate hollow cone, more or less deviating from, and
partially superposed upon the others.

Yet these felicities of explanation must not make us forget that the
chemical composition attributed to the first type of cometary trains
has, so far, received no countenance from the spectroscope. The emission
lines of free, incandescent hydrogen have never been derived from any
part of these bodies. Dissentient opinions, accordingly, were expressed
as to the cause of their structural peculiarities. Ranyard,[1277]
Zenker, and others advocated the agency of heat repulsion